Experimental studies will be undertaken to explore aspects of the pathophysiology of three clinical conditions common to infants and children: interstitial nephritis, ureteral obstruction, and renal transplatation. 1. The mechanism causing a decrease in GFR in patients with interstitial nephritis is unknown. We will explore this problem in an immunological model of tubulo-interstitial disease secondray to antitubular basement membrane antibody in nephron GFR, renal blood flow, filtration reaction, and fractional reabsorption of water and sodium. 2. A guinea pig model of hereditary congenital ureteral obstruction will be utilized to determine functional changes in the obsructed kindney and reversibility with relief of obstruction times post-natally. In other studies, ureters will be obstructed post-natally to determine effects on ureteral morphology and function and reversibility of these lesions. 3. Renal transplantation utilizing infant rats as donors and adult rats as recipients, will be done, as a model of the clinical situation in which children serve as donors for adult patients. The ability of these kidneys to grow and adapt functionally will be studied.